Who first discovered
gypsum?
What
causes the banding in malachite?
How do deposits
of gold form?
What causes the play of
color in opal?
What
is asbestos?
Who first discovered gypsum?
Gypsum is one of those minerals that is very useful and has been known from antiquity. Plaster made from gypsum is known from 9000 years ago from Syria and Egyptians used burnt gypsum 5000 years ago. For a brief history see this site.
Some of the first descriptions in surviving literature were by Theophraste (372-287 BC) and other ancient Greek and Roman writers.
The original name spelled in Greek is ( γεψοσ)
was applied primarily to calcined gypsum or plaster. Who initially named the material is lost to history.
What
causes the banding in malachite?
There is a significant color change possible for malachite when one looks at different crystal sizes. The lighter bands tend to be fibrous in nature, while the darker areas are crystals of larger size. I have a chunk of malachite from Zaire and the lighter areas are fibrous, while the darker areas consist of crystals that are much larger.
The streak of malachite is light green ( the color of small pieces of the material) while crystals are usually dark green to almost black. The specific gravity of the fibrous habits range down to 3.6 while crystals are 4.0. There is not much evidence for any major substitutions in the chemistry of malachite.
There can be changes in the growing conditions in the veins which would result in the inclusion of foreign minerals, or even inclusions in the crystals. The changes in chemical conditions in the veins could also cause the nucleation of additional crystals giving rise to the fibrous habit.
J. S. White found that the variation in color is due to changes in grain size, with the dark
bands consisting of large crystals, while the light bands were composed of smaller grains. White, J. S. 1991. Chemistry
of crystal size. Lapidary Journal 45:32-36,38.
How do deposits of gold form?
Gold is an element that is dispersed throughout the crust ( 3.1 ppb [parts per billion]) and is even found in seawater ( 0.05 ppb). To economically recover gold, it has had to have been concentrated by some means. The major way this is accomplished is by dissolving in hot water solutions that percolate through rocks. The solubility of gold is enhanced by chloride ions. When this hot water encounters changed conditions ( such as cooling or mixing with other waters ) the gold can
exceed it's solubility in water and be precipitated. The water circulation about intrusions or extrusions of volcanic rocks often provides the energy to drive this process.
Once these veins are formed, weathering of the rock frees particles of gold. Since gold is much heavier than most other minerals, it is much more difficult to move by running water and is concentrated near the places it weathered out of the rocks. It becomes concentrated by the mechanical weathering and produces placer deposits ( the black sand - magnetite - is also heavier than most other minerals and is found associated with the gold).
A third way of concentrating the gold is by biological activity. Some bacteria are known to be able to concentrate the gold from dilute solutions.
A web site that has a lot of information on gold is The
Gold Institute
What causes the play of color in opal?
Opal is a name for a number of minerals that are silicon dioxides with variable amounts of water. It is more crystalline than glass and less crystalline than quartz and it's high temperature and pressure phases.
Opal can grow in a number of different ways. It can rarely precipitate out of vapor in volcanic
fumarole deposits. It is the major silicon mineral created by biological processes.
Opal most commonly is precipitated from an aqueous solution when the ability of water to dissolve silicon is
exceeded. Silicon can dissolve into water from minerals at the surface and be carried down to the groundwater.
Precious opal gemstones are created when the opal forms small spheres of the mineral in a gel and settles to form a natural diffraction grating. Common opal or potch consists of particles that are either of varying size or not spheres. Etching of precious opal indicates spheres that consist of a small core surrounded by concentric rings of opal. The Australian opal is believed to have the silica dissolved from feldspar and concentrated by evaporation at the water table. The opal is deposited in cracks in the rock.
If you are interested in further reading, an article by Sanders and Darragh on "The microstructure of precious opal"
(Mineralogical Record 1971 vol 2 #6 pp 261-268) is excellent. There are also several chapters on opals and other silica minerals in the MSA's reviews in mineralogy vol 29 "Silica" 1994.
What is asbestos?
Asbestos is a commercial term for a number of minerals that have a fibrous or an acicular (needle - like habit). These include chrysotile - a serpentine mineral (a hydrous phyllosilicate) and the following amphiboles; amosite - a variety of gruenerite, crocidolite - a variety of riebeckite, fibrous anthophyllite, and fibrous tremolite and actinolite. Chrysotile was the most important mineral used in 95% of the asbestos. Tremolite and actinolite were not ever comercially important and anthophyllite is not now comercially important.
The first use of anthophyllite from Finland was as a strengthener for pottery from 2500 B.C. The Romans used asbestos for oil lamp wicks and as cremation cloths. The principal uses in the US were for cement pipe, flooring products, friction materials, and roofing products.
The major health problem with the material is that if the acicular needles of asbestos get into the atmosphere, they can cause tumors to form in the pleura of the lungs.
The Mineralogical Society of America has published several Reviews in Mineralogy that would be of interest for anyone interested in learning about asbestos. These include volume 9A "Amphiboles and other hydrous pyriboles - Mineralogy" and Volume 28 "Health effects of mineral dusts". These can be ordered from this site or can be found in most University libraries or obtained through interlibrary loan.
A good web site with additional information is the OSHA web site.
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